Protobranchia (500 - 5000m)
Shell height greater than or equal to length, subcircular to trigonal [c1, c2, c3], ligament internal in an anteriorly directed (opisthodetic) chondrophore [Nuculoidea & Pristiglomoidea]
Shell longer than high, frequently subrostrate or rostrate, [c4, c5, c6] ligament internal in a shallow resilifer or external. [Nuculanoidea]
Shell subcircular or obliquely oval, hinge with few posterior teeth subparallel to hinge plate [c7].[Pristiglomatoidea]
Shell triangular to subovate, hinge with numerous, prominent erect teeth [c.8] [Nuculoidea]
Internal margin denticulate [c11]
Internal margin smooth [c12]
Triangular distinctly higher than long [c13], marked decussate sculpture, commarginal ridges becoming stronger towards margin. (bathyal)
Nucula tumidula
Shells distinctly higher than long, triangular or rhomboidal
Shells longer than high subtriangular or subovate
Shell subovate, posterior truncated [c17], surface microscopically granular [c18]
Ennucula granulosa
Hinge teeth few, elongate, subparallel with hinge plate [c20].
Hinge teeth pointed erect
Shells subrostrate with a posterior ventral sinus
Shells rostrate or subovate to oval
Shells with a sharply pointed dorsally situated posterior, ligament both interior and exterior, gut loop multi-coiled.
Shells with a pointed, medially or downwardly angled posterior, ligament interior, gut loop simple or multi-coiled.
Shell thick becoming tumid, sculpture of fine commarginal ridges.
Ledella ultima
Shells normal thickness, sculpture of weak radial and commarginal ridges, periostracum pustulose.
Ledella pustulosa
Minute (<1.5mm), rounded tumid shells
External ligament visible close to beaks
Ligament internal not visible in joined valves.
Shell subovate to elliptical, posterior pointed
Thin shells with weak sculpture, subovate to oblong.
Yoldiellinae
